CAPE COD GEOLOGY 



87 



has elapsed since the outer islands were separated from the mainland by post- 

 glacial submergence is shown by the peculiar species of field mouse on Muskeget 

 Island, now a mere shoal between Nantucket and Marthas Vineyard. A very 

 similar example of variation under long isolation is seen in the mouse of Block 

 Island. Dense forests spring up on the stagnant margin of the ice of Alaskan 

 glaciers that approach a coast bathed by a warm oceanic current and a moist 

 atmosphere. Such forests may have grown along the ice front of southern New 

 England, and plants and animals may thus have bridged directly over the gaps 

 between lands that are now separated by arms of the sea. 



Long after the islands had become separated from one another and the 

 mainland, the American Indian may have established himself in the district. 

 Although few of his shell heaps are found on the present coasts, many of them 

 may have been lost by the encroachment of the sea upon the land. 



MINERAL RESOURCES 



The valuable mineral resources of Cape Cod and the New England Islands 

 are the Cretaceous white clay of Marthas Vineyard, the black iron sand of Block 

 Island, and the beach cobbles that are suitable for use as paving stones. An 

 attempt was made in 1891 to obtain capital to work the lignitic clay of Gay 

 Head x but the futility of the attempt was promptly shown. 



The selectmen of certain towns in the islands have from time to time sold 

 cobblestones from the beaches to contractors for paving streets in near-by cities, 

 but usually the inhabitants of the islands have quickly perceived the consequent 

 danger to the shore lands, and there is now little or no commerce in these essen- 

 tial defenses of the coast. 



The remoteness of Cape Cod rather than the absence of a grade of glacial 

 sand suitable for use as molding sand has prevented the exploration of the 

 region for supplies of this material. Such sand is likely to lie above the blue clay 

 on the north shore of Cape Cod near West Barnstable and on the islands at 

 what may be the horizon of the Jacob sand, but that formation usually lies 

 under a cover of coarse gravel and sand which prevents the economical recovery 

 of the finer sand. 



Small beds of blue clay crop out along the shore of Pleasant Bay on Cape 

 Cod and along the south shore of Cape Cod Bay from West Barnstable to the 

 entrance of the Cape Cod Canal. At West Barnstable a brick kiln has been in 

 1 Boston Herald, 90, No. 22, Wednesday, July 22, 1891. 



